By now it's become widely accepted that green roofs can help reduce heating and cooling costs for buildings, and evidence is mounting that they can provide tangible benefits in other areas as well. The latest piece of information comes from New York, for a green roof constructed by ConEdison, the city’s electric utility. The study, conducted in partnership with the Columbia University Center for Climate Systems Research, reveals that green roofs could play a much larger than role than previously expected in helping cities find cost effective ways to deal with excess storm water. That, in turn, provides building owners with new opportunities to participate in urban sustainability planning.

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Stormwater Runoff and Combined Sewer Overflows

Stormwater runoff from urban areas is is significant source of pollution in nearby waterways. In older cities with combined sewer systems like New York, the problem is compounded because the normal flow of sanitary waste from buildings commingles with stormwater and snow melt from streets. Most treatment plants have enough capacity to handle some of the excess, but in heavy weather some of the excess flow has to be shunted directly into nearby waterways. In recent years the city has employed a variety of strategies to reduce these events, called "combined sewer overflows." This includes the construction of large — and expensive — underground holding tanks.

The Green Roof Solution

New York has also explored the use of green roofs to trap excess stormwater, but previous studies suggested that this was not a cost-effective approach compared to other solutions. For this reason, the 2008 version of the city's sustainability master plan, PlaNYC, gave green roofs an unfavorable cost-benefit ranking in terms of reducing combined sewer overflows. However, the new Con Edison green roof study (pdf) reveals just the opposite. The researchers determined that the roof was trapping about 22 times more stormwater on an annual basis than was previously thought possible, making it the most cost-effective solution available.

The Green Roof Advantage

The researchers point out that green roofs have a distinct advantage over holding tanks and other infrastructure. Holding tanks simply capture excess flow, which is eventually sent to a treatment plant. In contrast, the excess flow captured by green roofs eventually evaporates or is absorbed by its vegitation, so it never enters the treatment system at all. That helps to reduce energy consumption and other costs related to treatment systems.